4.3.3 Create Your Own Plugin

Making a plugin in Crystal Space is not very hard but nevertheless
there are still a few issues that are often forgotten. Here in this
article we show you how you can write a simple plugin and use
it in your application.

Defining your Plugin API

The first thing that you need to do when making a plugin is to define the
API for it. The API is what your application is going to use to talk
to the plugin. It is the interface to the plugin so it is very important to
get this right. In the Crystal Space framework the Shared Class Facility
(see section Shared Class Facility (SCF)) is used to define the API. With this facility you create an
abstract interface containing only the methods from the API. An abstract
class in C++ means that all methods are pure virtual. This means that no
implementation is given; only method declarations. The implementation will
come later in the code of the plugin.

This concept is completely analogous to the Java interface mechanism. The
advantage of using this paradigm is that you have a clear separation between
the API and the implementation. This allows one to easily replace an
implementation of some API or even provide multiple implementations (for
example, the Null and OpenGL renderers are two implementations of the same
3D rendering API).

The above text should be put in a header file. Let's put it in
‘myapi.h’.

First we include ‘csutil/scf.h’. This is a Crystal Space header
for SCF which we need to get the definition of ‘iBase’ and the
definition of the SCF_INTERFACE() macro.

Then we declare ‘csVector3’ as a class. We do this so that we
can later use ‘csVector3’ as a parameter in one of the API methods.
We do not need the complete definition of ‘csVector3’ since we
are going to define the method so that it passes the vector by
reference.

In the interface declaration we use the SCF_INTERFACE() macro to
define the version of this interface. This versioning can be used to query for
specific versions of an interface. This can be useful later when you
want to extend the API without breaking existing apps. The version
has three parts: major, minor, and micro.

Finally we define the API by making a structure that inherits from
‘iBase’. We use ‘struct’ instead of ‘class’ simply because, for
structures, the default visibility is ‘public’ instead of ‘private’
as for classes. This is just a convenience. There is no other difference
between a ‘struct’ or a ‘class’ in C++. Note that you have to use
virtual inheritance for SCF to work properly.

The name ‘iMyApi’ is not random. Crystal Space uses this naming
convention (starting a name with ‘i’) for SCF interfaces so that it
is easy to see that they refer to SCF interfaces.

We inherit from ‘iBase’ because it is the basis of all SCF
interfaces. All SCF interfaces must inherit from ‘iBase’ either
directly or indirectly. This will ensure that we have reference counting (more
on that later) and also takes care of the other internal SCF issues.

In that structure we define two methods: DoSomething() and
GetSomething(). Note that every method is defined as follows:

virtual ... = 0;

The ‘= 0’ means that we will not give an implementation here. The
implementation will be provided by the plugin (see later).

Note that it is good practice to use ‘const’ wherever applicable. In the
declaration of GetSomething() we added ‘const’ at the end to
indicate that this method will not change the object. This is useful
for a number of reasons:

It serves as documentation for the API reader.

A good compiler might be able to do some optimizations if it knows that the
method will not modify the object.

Creating the Plugin Implementation (header)

After you defined the API for your plugin it is now time to actually
make the plugin implementation. First you define a header called
‘myplug.h’ with the following contents:

This requires a little explanation. The Crystal Space plugin framework
requires that every namedSCF class which will be requested by name
from a plugin module via the Crystal Space plugin manager/loader must
implement the ‘iComponent’ interface. This interface has a single method,
Initialize(), with which the class will be initialized after it is
instantiated. This gives the instance a chance to perform various
initialization operations and it also provides the instance with a pointer to
the global object registry.

But, our plugin also needs to implement its own native ‘iMyApi’ interface.
So here is a situation where the same class needs to implement two interfaces
at the same time. By using the ‘scfImplementation2’ templated class
we can easily declare the class ‘MyPlugin’ to implement both
‘iComponent’ and ‘iMyApi’.

In the declaration of ‘MyPlugin’ we then have to implement all
functions from both ‘iComponent’ and ‘iMyApi’. To do that
the method declarations from both interfaces are repeated here but the
‘= 0’ is removed. This means that we'll actually give a concrete
implementation here.

Note that ‘MyPlugin’ needs a constructor that accepts an ‘iBase*’
parameter. Otherwise SCF will not be able to intantiate this class.

Creating the Plugin Implementation (source)

Now we create the main source file containing the implementation
of our plugin. Let's call this ‘myplug.cpp’:

The first macro is CS_IMPLEMENT_PLUGIN. This indicates to the
Crystal Space framework that this module will end up as a plugin
(as opposed to an application or library). On some platforms this
actually makes a difference; on others it does not. For best portability,
you should use this macro in exactly one C++ file within each plugin
module. (Note: If your project uses the Jam-based CS build system
please also see plugins-in-cs-build-system.)

The SCF_IMPLEMENT_FACTORY() says that C++ class ‘MyPlugin’
represents an SCF factory which allows SCF to instantiate objects of
this class. In addition to some other administrative tasks, this macro defines
a function capable of instantiating an object of class ‘MyPlugin’. Note
that one plugin module can in fact define several distinct named SCF
classes. In that case you need multiple SCF_IMPLEMENT_FACTORY() lines;
one for each exported SCF class.

In the constructor of ‘MyPlugin’ you must also call the constructor
of the templated superclass by using scfImplementationType(). The
first parameter to scfImplementationType() is always ‘this’.

The rest of the plugin is very straightforward. It is important
to realize that you should do most initialization of the plugin
in the Initialize() function and not in the constructor. The reason
for this is that, at construction time, you cannot depend on the entire
Crystal Space framework being ready. Also when Initialize() is
called you get a pointer to the object registry which is essential
for locating other modules and plugins loaded by the Crystal Space
framework.

Telling SCF About Your Plugin

SCF discovers plugins automatically and dynamically. It determines which
plugin modules implement which SCF classes by consulting meta-information
associated with each plugin. The meta-information file for your plugin must
have the same basename as your built plugin module, but with extension
‘.csplugin’. For instance, if the example plugin is built with the name
‘myplugin.dll’ (Windows) or ‘myplugin.so’ (Unix), then the associated
meta-information file should be named ‘myplugin.csplugin’. At build-time,
the meta-information may be embedded directly into the plugin module if
supported by the platform and if embedding is enabled. If not, then then the
‘.csplugin’ file will be laid down alongside the built plugin module.

The meta-information file is a structured XML-format document, and can
contain any information relevant to the plugin module; it is not limited only
to SCF information. SCF itself expects to find a node named
<scf>, which contains SCF-related information about the plugin
module.

Each named SCF class exported by the plugin should be presented in a
<class> node within the <classes> group. Each class has a
<name>, which is the SCF name of the class; an
<implementation>, which is the name of the C++ class implementing the
SCF class; a <description>; and optionally a <requires>
node, which lists the other SCF classes upon which this class depends.
Any number of classes may appear in the <requires> group. If your
plugin depends only upon a certain type of class, rather than a specific
SCF class, then you list only the prefix portion of the desired class
type, as shown in this example (where we desire any 3D renderer).

Compiling the Plugin

Depending on the development tools that you use, you should refer to one of the
HOWTO's on the subject of building an external Crystal Space module.

Loading the Plugin in Your Application

First, include the header defining the API of the plugin:

#include <myapi.h>

Do not include the ‘myplug.h’ header file since it is
implementation specific and you should not use the implementation of the plugin
directly. Doing this invalidates the entire reason to use plugins in the first
place.

To load the plugin there are a few possibilities. First, you
can load the plugin manually using csLoadPlugin like this:

This will get the plugin manager from the object registry. This is the module
that is responsible for loading and unloading plugins. The code then uses the
plugin manager to load your plugin. Note that this can fail. You should always
check the returned value to see if it is valid.

Another way to load the plugin is through RequestPlugins(), which
is called at initialization time:

This way has several advantages. First, it allows the user to override your
plugin at the command line or in the configuration file (if your program has
one). In cases where there are multiple implementations for the same API
this can be an important consideration. It is by doing this, for example, that
it is possible to switch between Null and OpenGL renderers with the
command-line ‘--video=’ option, or via configuration file.

Secondly it registers the plugin with the object registry so that
it is easier to find your module later. This also allows other plugins
to find your plugin by doing a query on the object registry.

Using the Plugin in Your Application

After loading the plugin you can use the plugin simply by
calling the methods defined in the API:

Plugins in the Crystal Space Build System

When writing plugins for a project that uses Crystal Space's Jam-based build
system invoking the CS_IMPLEMENT_PLUGIN macro is not needed.
A source file with an invocation will automatically be provided by the build
system.

Neglecting to invoke CS_IMPLEMENT_PLUGIN if needed or wrongly invoking
it when the build system already does will cause build errors; see the next
paragraphs.